1. Field of the Invention
This specification relates to a structure of contacts for an air circuit breaker, and more particularly, a structure of contacts for an air circuit breaker, in which a movable contact arm can stably be brought into contact with a fixed contact arm by changing an electromagnetic repulsive force generated between a movable contact and a fixed contact.
2. Background of the Invention
In general, an air circuit breaker is a circuit breaker which is installed on a top of a low pressure power distribution system, and has functions of maintaining a conductive state for a preset time when a fault current is generated on a circuit due to short-circuit, overload, electric leakage, etc. and breaking the circuit when the fault current remains after the preset time.
FIG. 1A is a schematic view illustrating a separate state between contacts of an air circuit breaker according to the related art, and FIG. 1B is a schematic view illustrating a contact state between the contacts of the air circuit breaker according to the related art.
As illustrated in FIGS. 1A and 1B, the air circuit breaker 100 according to the related art includes a fixed contact arm assembly 20, a movable contact arm assembly 30 relatively movable with respect to the fixed contact arm assembly 20, a switching mechanism 40 for relatively moving the movable contact arm assembly 30 with respect to the fixed contact arm assembly 20, and an arc-extinguishing unit 50 for extinguishing arc generated during opening or closing of the air circuit breaker 10.
The fixed contact arm assembly 20 includes an upper terminal 21 connected to a power source side circuit, a fixed contact arm 22 fixed to the upper terminal 21 to receive power, and a fixed contact 23 provided on the fixed contact arm 22.
The movable contact arm assembly 30 includes a lower terminal 31 connected to a load side circuit (not illustrated), an connect terminal 31a disposed on the lower terminal 31, a cage 32 made of an insulating material and having one end rotatably installed on an air circuit breaker housing (not illustrated) through a rotation shaft 38, a movable contact arm 33 disposed on the cage 32 to be rotatable centering on the rotation shaft 38, a contact spring 34 disposed between the movable contact arm 33 and the cage 32 to press the movable contact arm 33 toward the fixed contact arm 22, a movable contact 35 disposed on the movable contact arm 33 and brought into contact with the fixed contact 23 when the movable contact arm 33 is rotated toward the fixed contact arm 22, a wire 36 provided between the movable contact arm 33 and the connect terminal 31a to allow a current flow between the movable contact arm 33 and the connect terminal 31a, and a link 37 having one end connected to the cage 32 and another end rotatably connected to the switching mechanism 40.
The switching mechanism 40 is a mechanical device that applies a driving force through the link 37 such that the movable contact 35 provided on the movable contact arm 33 is brought into contact with or separated from the fixed contact 23 provided on the fixed contact arm 22.
The arc-extinguishing unit 50 includes a plurality of grids (not illustrated) disposed between the fixed contact 23 and the movable contact 35 (hereinafter, the term “contact” is also used with respect to the fixed contact and the movable contact for the sake of representation) to extinguish arc which is generated between the fixed contact 23 and the movable contact 35 when the movable contact 35 is brought into contact with the fixed contact 23, or specifically, separated from the fixed contact 23, and an arc runner 51 disposed on the fixed contact arm 22 to induce the arc generated between the fixed contact 23 and the movable contact 35 toward the arc-extinguishing unit 50.
Hereinafter, a closing operation of the air circuit breaker 10 having the configuration will be described with reference to FIGS. 1A and 1B.
During a closing operation of the air circuit breaker 10, the switching mechanism 40 rotates the cage 32 illustrated in FIG. 1A through the link 37 in a counterclockwise direction centering on the rotation shaft 39.
When the cage 32 is rotated in the counterclockwise direction, the movable contact arm 33 which is rotatably disposed on the cage 32 is rotated in the counterclockwise direction centering on the rotation shaft 38. Afterwards, a contact surface 35s of the movable contact 35 is brought into contact with a contact surface 23s of the fixed contact 23, and thereby the rotation of the movable contact arm 33 is stopped.
However, the cage 32 is more rotated in the counterclockwise direction by a preset range due to the switching mechanism 40. Accordingly, as illustrated in FIG. 1B, the contact spring 34 disposed between the movable contact arm 33 and the cage 32 is compressed.
The compressed contact spring 34 elastically presses the contact surface 35s of the movable contact 35 of the movable contact arm 33 onto the contact surface 23s of the fixed contact 23, and accordingly, a current flows between the fixed contact 23 and the movable contact 35.
In the air circuit breaker 10 having such construction and performing the closing operation, while the current flows between the movable contact 35 and the fixed contact 23 in response to the movable contact 35 being brought into contact with the fixed contact 23, a direction of a current that flows from the contact surface 23s of the fixed contact 23 and a direction of a current that flows to the contact surface 35s of the movable contact 35 are opposite to each other. Accordingly, an electromagnetic repulsive force is applied between the fixed contact 23 and the movable contact 35.
The electromagnetic repulsive force tries to rotate the movable contact arm 33 in a clockwise direction (i.e., a breaking direction) through the movable contact 35 centering on the rotation shaft 38, but a load of the contact spring 34 is applied opposite to the electromagnetic repulsive force, which results in maintaining the contact state between the contacts 23 and 35.
However, when a great electromagnetic repulsive force is generated between the contacts 23 and 35 due to a heavy current such as a fault current or abnormal current, the electromagnetic repulsive force becomes stronger than the load of the contact spring 34 and thereby rotates the movable contact arm 33 in the clockwise direction. This may be likely to separate the movable contact 35 from the fixed contact 23.
However, the air circuit breaker 10 is the circuit installed on the top of the low pressure distribution system. Thus, in order to ensure a time for a lower circuit breaker (not illustrated) located on a lower circuit to perform a breaking operation although such great electromagnetic repulsive force is generated between the contact 23 and 35 due to the heavy current such as the fault current or the abnormal current, it is required to maintain the contact state between the contacts 23 and 35 for a predetermined time (typically, 1 to 3 seconds).
If the contacts 23 and 35 of the air circuit breaker 10 are separated from each other by the electromagnetic repulsive force generated due to the fault current or abnormal current, a fault may be likely to happen in the lower circuit of the air circuit breaker 10.
Therefore, in the related art air circuit breaker 10, the contact spring 34 has no option but to be set to have great loads to sustain the electromagnetic repulsive force due to the fault current or abnormal current, as well as a rated current of the air circuit breaker 10, for a predetermined time.
However, when the contact spring 34 with the great loads is applied to the movable contact arm assembly 30, the loads of a closing spring (not illustrated) applied to the switching mechanism 40 should also increase proportionally. This may, however, bring about various problems, such as an increase in an impact between the contacts 23 and 35, an increase in abrasion between the contacts 23 and 35, a reduction of a number of times of breaking a circuit, degradation of durability of an air circuit breaker mechanism and the like.